Network traffic with center cutout [3/3]

Extends the settings for network traffic display. It is always
allowed, even if center cut out or centered clock.
One can select left/right and - if possible - center.

The left/center/right text strings are shared with the
status_bar_clock_position texts.

Change-Id: I5807b7822d6a65611abad104f820ce6a0cb93ee3